Krugman wrote in a paper that appears at the City University of New York website, where he nowteaches:

Trump’s tweet on German-US trade was, it goes without saying, deeply stupid and destructive. He obviously doesn’t get how the EU works – it’s a customs union, so there is no such thing as bilateral trade policy. He also thinks that bilateral trade balances are the test of fairness, which is all wrong.

It is correct as far as it goes but Krugie is not really a deep, deep thinker.

If Trump was a three-dimensional economic policy player, and if he wanted to wreak havoc amongst EU members, he could slap tariffs on individual countries within the EU and let them sit in their Brussels custom union meeting house and think about that for awhile. He doesn't have to play by their rules. It is possible by that move alone that Trump could break the EU---and Krugie would have to re-write his paper.